Electrolyte production of a protective coating on articles



Oct. 28, 1969 w. J. CAMPBELL. 3,475,2Q6

OLYTIC PRODUCTION OF A PROTECTIVE COATING ON ARTICLES Filed April 5,1967 3 Sheets-Sheet l Inventor Mu. MM J. CAMPMLL By M A Horneyzi Um. 28,1969 w. J. CAMPBELL ELECTROLYTIC PRODUCTION OF A PROTECTIVE COATING ONARTICLES Filed April 5. 1967 3 Sheets-Sheet 2 Inventor Mu/lqM'dCnmacu AHorneys m. 28, 1969 w. J. CAMPBELL 3,475,296

ELECT OLYTIC PRODUCTION OF A PROTECTIVE COATING ON ARTICLES Filed April5. 19s? a Sheetsheet a IIHIIIIHIIHIIIII]IIIIHIIIHHHHIIIIIHHIIIIIIHI I tnvenor I -l$ By Attorneys United States Patent ELECTROLYTE PRODUCTION OFA PROTECTIVE COATING ON ARTICLES William J. Campbell, 42 Hamilton Ave.,Glasgow S.1, Scotland Filed Apr. 5, 1967, Ser. No. 628,664

Claims priority, application Great Britain, Apr. 5, 1966,

15,036/ 66 Int. Cl. C23b 9/02 US. Cl. 204-58 9 Claims ABSTRACT OF THEDISCLOSURE A method of electrolytically producing a protective coatingon articles, particularly a method of anodising an aluminium cylinder byrotating only part of the periphery at any one time in an electrolytewhich flows at right angles to the direction of rotation in an annularpassage defined by a curved bafie which can be an electrode. The radiusof curvature of the baffle is adjustable so that, with cylinders ofdifferent diameters, the bafiie can be equispaced at all points fromeach cylinder to form an even coating. Cooling electrolyte progressivelyincreases in a direction away from the electrolyte entry to the passage.

This invention relates to the electrolytic production of a protectivecoating on articles by, for example, electroplating, electro-forming oranodising. For convenience of description, the invention will bedescribed in connection with the anodising of articles of aluminium oraluminium alloy.

The invention is concerned particularly, but not exclusively, with theanodising of large aluminium articles of cylindrical cross-section, suchas torpedoes.

It is well known that adequate agitation of the electrolyte, or of analuminium article in the electrolyte, is essential to the production ofa compact uniform anodic coating on the article and various methods havebeen proposed heretofore to provide this agitation condition. One suchmethod is described in my prior British Patent 716,554. Another and moretypical method is to inject air into the electrolyte from perforatedtubes at the bottom of an anodising tank. None of these methods,however, are entirely adequate for the production of an anodic coatingto a close tolerance of hardness over the periphery of a largecylindrical article.

If a large cylindrical article were to be completely immersedhorizontally in an anodising tank of dimensions not much greater thanthose of the article, it will be understood that air agitation from thebottom of the tank would result in the formation of a softer coating inrelatively still electrolyte on the top of the cylinder and said coatingwould become warmer, while the amount of current required would probablybe in excess of the rated capacity of the tank and its electricalequipment.

While the method of agitation described in my prior British Patent716,554 would theoretically be adequate for hard anodising a largecylindrical article located vertically in a circular cathode, and hasproved excellent in production for anodising cylindrical articles about8 inches in diameter and feet in length, such a method would requirelarge resources of power, pumping and refrigeration for very largecylindrical articles.

The primary object of the present invention is to provide an economicalmeans of anodising a varied range of large cylindrical articles having adiameter up to, for example, 5 feet and a length of 6 feet, to a closetolerance of maximum hardness over the entire periphery of the article.

The present invention is a method of electrolytically producing aprotective coating on an article, including the steps of continuouslyrotating the periphery of the article, which serves as an electrode, inan electrolyte in such wise that only a portion of the periphery isimmersed in the electrolyte at any one time, and causing the electrolyteto flow in contact with the immersed portion of the articlesubstantially at right angles to the direction of rotattion of saidperiphery.

Preferably the electrolyte is caused to flow through an annular passagedefined by the periphery of the rotating article and the inner wall ofan arcuate bafile.

A first embodiment of the present invention will now be described by wayof example with reference to the accompanying drawings in which:

FIG. 1 is a front view of the apparatus used in the process:

FIG. 2 shows the apparatus for longitudinally and vertically adjustingthe central pivot:

FIG. 3 is an end view of the rigid plate and the adjustable coppersheet:

FIG. 4 is part plan view showing the direction of electrolyte flow:

Referring to FIG. 1, a cylinder 1 is provided at each end with a centralpivot 2 which rests on non-conductive hearings or rollers 3longitudinally and vertically adjustably set at such a height above thebase of a tank 4 of electrolyte 5 that the pivots 2 are locatedsubstantially above the surface level of the electrolyte 5 and part onlyof the periphery of the cylinder 1 is immersed in the electrolyte 5.

The rollers 3 are suspended from cross channels 6 (see FIG. 2) which areadjustably mounted on upper side members 7 of the tank 21. The pivot 2at one end is operatively connected either directly or through auniversal transmission link 8 to a driving motor (not shown), preferablya variable speed motor. One of the pivots 2 has provision for a brushtype of electrical connection (not shown) to supply electrical currentto the cylinder 1 while it is rotating.

A hard rolled copper sheet 9 serving as a bafile is located in the tank4 to form an arcuate portion around but spaced from the immersed portionof the cylinder 1, each of the two longitudinal edges of the sheet 9being bounded by angle sections 10 of copper with adjustable cross links11 (see FIG. 3) at either end so that the radius of the arc of thecopper sheet 9 may be adjusted by shortening or increasing the effectivelengths of the end links 11. The copper sheet 9 is sufficiently flexiblefor the purpose and, as it is always cathodic when in use, it does notdeteriorate in an electrolyte of, for example, sulphuric acid and it canbe independently maintained in cathodic protection when the process isnot in operation.

One end of each of the angle sections 10 is extended to rest on avertical rigid plate 12 of polyvinyl chloride which is secured to thelowest point of the curved copper sheet 9 by a single semi-rotatablemounting 13 and which has a top edge curved so that in any radius of thecopper sheet 9 the sections 10 always rest on the plate 12 which is heldclose to the sheet 9 by projections 14 (FIG. 1)

on said sections close to the plate 12 on the face remote from thesheet.9. By this means one end of the segment formed by the curved sheet9 in any position is masked by the plate 12 which has an openingconnected to a pump (not shown) which is arranged to pump electrolyte 5to flow through said opening 15 and along the annular passage presentedbetween the copper sheet 9 and the cylinder 1.

This lowest point of the curved sheet 9 rests on the bottom of thecomparatively shallow acid resistant tank 4 which has an adjustable weir16 in the end of the tank 4 adjacent to the plate 12. The height of thisweir 16 controls the level of electrolyte 5 in the tank 4 before theelectrolyte 5 reaches a connection back to the suction 17 of the pump.The end of the curved sheet 9 remote from the plate 12 is open andsufiiciently spaced from the adjacent end wall of the tank 4 so as notto impede the flow of electrolyte 5 within the arc of the copper sheet 9and return flow of the electrolyte 5 along the outside of the coppersheet 9 back to the weir 16 and the suction 17 of the pump.

It will be understood that the flexible copper sheet 9 can be arrangedto maintain a substantially uniform annular passage between the sheet 9and the submerged portion of a large truncated cone by appropriateadjustment of the links 11 at both ends to different effective lengths.In this case, the lowest point of the sheet 9 at the end remote from theplate 12 would be raised from the bottom of the tank 4. As the level ofthe electrolyte 5 inside the curved sheet 9 is the same as the leveloutside the curved sheet 9 there will be no load on the sheet 9 to causedistortion from a substantially true radius in any position.

Although the invention described so far with the electrolyte 5 pumped atright angles to the direction of movemnet of the periphery of thecylinder 1 provides very vigorous and uniform agitation, the temperatureat the end of the annular passage remote from the plate 12 can, on alarge cylinder of considerable length increase considerably from thetemperature at the input end. Even some vaporization of the electrolyte5 from the unimmersed end of the cylinder remote from the plate 12 mayoccur and the hard anodic coating would then become considerably palerin colour and of progressively less hardness toward this remote end. Therotating cylinder 1 can in these circumstances become warmer to touch atthis remote end in the latter stages of the process when the power inputis at its maximum.

This inequality in the quality of the anodic coating is obviated ormitigated on a long cylinder 1 by arranging a longitudinal sparge tube18 above and parallel to the top of the unimmersed portion of thecylinder 1. The tube 18 has holes 19 of diameters decreasing in sizefrom the end of the cylinder 1 remote from the plate 12 to the other endand fresh cold electrolyte is introduced into the apparatus through thistube 18 to cool the cylinder 1. The larger holes at the end of the tube18 remote from the plate 12 cause a proportionately larger amount offresh electrolyte to be directly projected onto what would have becomethe warmer end of the cylinder 1 and a progressively smaller amount ofelectrolyte to be directed toward the end of the cylinder 1 adjacent tothe input end of the main electrolyte flow 5. The sparge tube 18 mountsslidably adjustable rubber sleeves 20 which can be used to control theflow from any of the holes 19 so as to provide for maintaining a uniformtemperature over the whole length of the cylinder 1.

It has been found where the inside and outside of a hollow cylinder arebeing simultaneously hard anodised that this external cooling of thecylinder had an almost identical effect on the inner surface of thecylinder. Although no fresh electrolyte was directed at the innersurface, an increased flow of fresh electrolyte applied externally fromthe sparge tube adequately prevented overheating of the hard anodiccoating forming on the in- Cit terior wall of the cylinder. Hollowcylinders are usually sealed at their ends if it is desirable to preventany electrochemical action on the interior surface.

On a cylinder of short length, or where the maximum power input is wellwithin the cooling capacity achieved by the cross flow method ofagitation, the sparge tube may be unnecessary and the fresh coldreplacement electrolyte which is always necessary to maintain a uniformtemperature in the tank can be introduced into the apparatus at any moreconvenient plate than through the sparge tube which usually necessitatesthe use of splash guards.

A bypass 21 from the pump can be used to pass electrolyte into anelevated heat exchanger (not shown) with a gravity feed of cooledelectrolyte hack to the anodising tank 4 in order to maintain a uniformtemperature of electrolyte 5 or a separate pump can be used for thispurpose to return cooled electrolyte from a heat exchanger at the levelof the apparatus.

In the electrolyte flow along the outside of the copper sheet back tothe pump, cooling coils may be immersed to control the temperature ofthe electrolyte as an alternative to the elevated heat exchangerreferred to in the preceding paragraph.

In place of the copper cathode sheet, a sheet of stainless steel orother metal, or a sheet of plastics material with straps of metalattached, may be used as the cathode.

In a second embodiment where the axis of rotation of the member is belowthe level of the electrolyte, the driving pivot is electricallyconnected by a very flexible woven wire mesh or braided cable alongsidethe submerged universal link to a point beyond the non-submergeduniversal link when it is electrically connected to the motor drivenspindle which is insulated from the motor and receives its electricalpower from a suitable brush connection. The whole arrangement isprotected as required from the electrolyte by a flexible rubber sleeve.

Both embodiments accommodate rotating articles of different diametersbut where it is required only to process one given diameter the curvedsheet which constitutes both the cathode and the annular passagerestricting wall would be rigid and constitutes also the outer wall ofthe process apparatus. It would be entirely of metal or of metal partlycovered to reduce the effective electrode surface, or it could be ofplastics or rubber with metal strips constituting the effectiveelectrode.

Although these embodiments have been described in connection with thehard anodising of large aluminium cylinders of different diameters itwill be appreciated that long large articles of hexagonal or similarsection could be treated in the same manner but some of the adjustableelements to accommodate the difierent diameters may be excluded when thecylinders being processed are uniform in diameter.

The above described method of anodising is applicable also to othermethods of electrolytic production of protective coatings, such aselectro-plating or elcctro-forming.

I claim:

1. A method of electrolytically producing a protective coating on anarticle, including the steps of continuously rotating the periphery ofthe article, which serves as an electrode, in an electrolyte in suchwise that only a portion of the periphery is immersed in the electrolyteat any one time, and causing the electrolyte to flow in contact with theimmersed portion of the article substantially a right angles to thedirecion of rotation of said periphery through an annular passagedefined by the periphcry of the rotating article and the inner wall of aflexible arcuate baffle, means being provided for adjusting the radiusof curvature of the baflie.

2. A method as claimed in claim 1 in which the baffle constitutes anelectrode.

3. A method as claimed in claim 1, in which the axis 5 of rotation ofthe article is located above the level of the electrolyte.

4. A method as claimed in claim 1, in which the axis of rotation of thearticle is located below the level of the electrolyte.

5. A method as claimed in claim 1 in which the baffle constitutes thewall of a processing tank.

6. A method as claimed in claim 1, in which the electrolyte isrecirculated outside the baffle and the level of the electrolyte withinthe baflle is substantially the same as the level of the electrolyteoutside the baflle.

7. A Inehod as claimed in claim 1, in which cooling electrolyte isprojected upon the unimmersed portion of the rotating electrode.

8. A method as claimed in claim 7, in which the volume of the coolingelectrolyte applied to said unimmersed portion of the article increasesprogressively in 6 the direction away from the end of the annularpassage to which electrolyte is admitted.

9. A method as claimed in claim 1, in which the method is anodising.

References Cited UNITED STATES PATENTS 1,862,745 6/ 1932 Fuller et al2049 2,076,909 4/1937 Miller et a1 20425 2,155,392 4/1939 Ballard204-112 2,905,604 9/ 1959 Kennedy et a1. 204-5 6 JOHN H. MACK, PrimaryExaminer 15 R. L. ANDREWS, Assistant Examiner US. Cl. X.R.

